About electric double layer capacitors using an electric double layer formed at interface of a polarizable electrode and an electrolyte surface, the demand thereof as memory backup electric power sources has been rapidly expanding in recent years. Moreover, attention has been paid to the application thereof to articles for which a high capacity is required, such as a power source for a fuel cell mounted vehicle.
An electrode for an electric double layer capacitor has a structure obtained by molding an electrode-forming composition comprising a carbon material as an active material, a binder and an optional electroconductivity additive to prepare an electrode layer, and then laminating the electrode layer on a metal foil, metal mesh or the like as a current collector.
As the method for molding the electrode layer, press-molding is known. For example, Japanese Patent Application Laid-Open (JP-A-) Nos. 63-107011 and 2-235320 each suggest a method of press-molding an electrode-forming composition comprising fine carbon particles, a fluorine-containing polymer such as polytetrafluoroethylene (PTFE), and a liquid lubricant to form an electrode layer. JP-A No. 9-306798 also suggests a method of integrating a kneaded product of activated carbon and PTFE as a binder with a current collector electrode made of metal and press-molding the integrated product to form an electrode.
However, in the case of using PTFE as a binder, it is necessary to pre-knead the electrode-forming composition to turn PTFE into a fibrous form and it is also necessary to remove a kneading auxiliary agent added at the time of the pre-kneading. In such a way, a problem that the process becomes complicated is caused. When PTFE is pre-kneaded, regions made into a fibrous form and regions not made into a fibrous form are generated; therefore, when an electrode layer in a thin film form is formed, the surface of the resultant easily becomes uneven. Thus, the strength of the electrode may be insufficient or the performance of the electric double layer capacitor to be obtained may not be sufficiently good.
As a method using a binder other than PTFE, suggested is also a method of molding activated carbon having a specific particle diameter and plastic powder having a specific particle diameter into a plate form at a temperature near to the melting point of the plastic (JP-A No. 4-22062). Suggested is also a method of mixing activated carbon, an electroconductive material, and binder powder made of a thermoplastic resin or B-stage thermosetting resin in a powdery state to obtain mixed powder, and press-molding the mixed powder to form an electrode (JP-A No. 63-151010). However, according to these methods, the resultant electrode is insufficient in flexibility. Thus, when the electrode is wounded and then put into a container, its electrode layer may be cracked or may fall away from the current corrector. The performance of the electric double layer capacitor to be obtained is also insufficient.
As a method using an elastomer as a binder, JP-A No. 62-16506 suggests a method of mixing a latex and activated carbon, dehydrating the mixture, grinding and granulating the resultant aggregate, and then press-molding the granulated product. Furthermore, JP-A No. 8-250380 suggests a method of press-molding a mixture obtained by mixing a solution of styrene butadiene rubber or acrylonitrile butadiene rubber in xylene with activated carbon and then drying the resultant. However, the methods described in these documents have complicated steps, and the performance of the electric double layer capacitor to be obtained is also insufficient.